There are indications of large differences in performance across methadone maintenance programs, as measured by patients' illicit drug use during treatment and retention in treatment. One benchmark study (Ball and Ross 1991) found program quality factors to be most highly related to patient outcomes, several other studies have concluded that dosage policy is most important, and most studies have not examined treatment environment at all. In general, the relation of program quality/treatment environment variables to outcomes has not been well-addressed in existing methadone evaluations. The major completed methadone outcome studies also are out-of date and relied too much on fallible self-reports of illicit drug use. The purposes of this study are: * To describe the differences in patient background and admission characteristics; in program quality/treatment environment; in service utilization; and in patient outcomes across a diverse sample of methadone maintenance clinics. Outcomes are defined as: type/severity of illicit drug use during treatment, length of retention in treatment, and type of discharge from treatment. * To determine the effects of methadone program quality/treatment environment factors on patient outcomes, controlling for differences in patient characteristics. The study will attempt to replicate and cross- validate the important Ball and Ross (1991) findings with a larger, independent sample of methadone maintenance clinics. * To make concrete recommendations for improving patient outcomes and the cost-effectiveness of methadone treatment. The method would be a retrospective longitudinal cohort study of 2000 admissions during 1989/1990 to 20 separate clinics affiliated with 11 methadone maintenance programs in New York City; the cohort will be followed-up retrospectively for 3 years, through 1992/1993. Available automated data are: patient social history/admission data, methadone dosage and urinalysis results, and discharge data. Service provision/treatment process data will be abstracted on-site from clinical/medical charts. Program quality data (e.g., organization, structure, staffing management, service mix, service delivery mechanisms, staff satisfaction) will be obtained through a clinic site visit protocol.